This invention is related to automatic weighing machines of the check weigher type as used in the packaging industry and more specifically those types of check weighers which incorporate the use of an automatic package transport system for moving packages across a weighing platter in a steady stream whereupon the weight of each package relative to an acceptable weight range is determined. A load cell or weigh cell is connected to the weighing platter for electromechanically determining weight.
Weighing machines have been incorporated into the production facilities in the packaging industry for many years. These machines are used to weigh a final product package to determine if that package contains the net weight specified on the label. Such weighers are used in the pharmaceutical industries, the canning and bottling industries and any other types of industry where goods are packaged and sold by weight. Prior, early vintage check weigher machines were simple scales which were loaded and unloaded by an operator. An acceptable weight, plus or minus a predetermined tolerance, was normally marked on the scale so that the operator could readily check a given package to determine if its weight was within the specified range.
Improvements in check weighing apparatus incorporated continuous feed and discharge features, including automatic feed and discharge procedures for the weighing of the packages. Weights were determined while a package was momentarily on a weighing platter. In such systems an input feed belt deposited a package on a weighing platter. The momentum carried the package across the platter and onto the discharge belt.
Very often weighing platters have included rollers which allow a package to slide across the weighing platter without drastically jarring it. Chains, belts, chutes and other means have been employed for feeding and discharging the weighing platter.
With the advent of such dynamic "on-the-fly" type check weighing apparatus, a dominant problem of accurately determining the weight of the package as a package travels rapidly across the weighing platter developed. This problem included more than just the rapid reading of a weight. The weighing platter normally, with this type of operation, is subjected to an impulse as the package first hits it. This impulse causes an oscillation in the weighing platter and the weigh cell structure. Such an oscillation takes a fixed period of time to dampen out. Before this happens a package may have moved off of the platter.
With prior art check weighers it was determined that an acceptable level of oscillation of "error" would be permitted in order to obtain a high throughput rate thus reducing the transient time that a package is on the weighing platter. Normally, therefore, the package would be weighed, i.e., the weigh cell reading would be sampled, when the package has reached about three quarters to seven-eighths of the distance across the weighing platter but while still remaining completely on the weighing platter.
Given this design criteria, prior art check weighers would trigger a sample reading of the weigh cell upon a position or location of a package on the weighing platter. Mechanical gate "fingers", photoelectric cells and the like have been taught as triggering devices for sampling the weigh cell output as a package reached a certain spot across the weighing platter.
Recently, electronic techniques have been applied to the check weighing art. Electronic scales and weighing systems have been developed which provide a relatively rapid read out in digital format. A particularly useful system includes the connection of a digital conversion circuit to the weigh cell which converts the analogue voltage output from the weigh cell into a digital output signal corresponding to the weight of the package. Such a system is taught by Kavanagh, et al, U.S. Pat. No. 4,049,068.
Kavanagh teaches the classic positionally-triggered read out scheme wherein retroreflective photosensors are used in conjunction with reflectors to control the timing of the weight sampling and registering system upon the movement of incoming articles for triggering a sample reading from the weigh cell, i.e., the weight reading.
Kavanagh also teaches a method of establishing a weight of a package by subtracting the weighing platter laden weight from the weighing platter unladen weight for each and every package passing through a check weigher. This method eliminates the need for providing a stable "zero" reference when the weighing platter is unladen. However, it limits the throughput rate and therefore the capacity of the machine by requiring a sampling of unladen weighing platter weight between each package.
Traditionally electronic check weighing circuits have been "zeroed", statically, by using the circuit output for a stable, quite, unladen weighing platter as reference. This is necessary because measuring systems using sophisticated electronic equipment including digital circuitry must concern themselves with the normal electronic drift in the equipment which processes the weigh cell signal. This circuitry drift which introduces error and causes inaccuracies in output readings is a normal occurrance with operating hours, changes in ambient temperature and humidity, repeated subjection to start-up and turn-off transients and many other factors. However, the static zeroing technique of an unladen platter may prove unsatisfactory for dynamic weighing conditions.
While check weighers have become considerably more sophisticated than ever before, room for improvement still abounds.
A better technique for dealing with weighing platter transients is needed to obtain more accurate output readings. Automatic dynamic zeroing is also desirable. Moreoever, it is desirable to eliminate position, triggered sampling of the weigh cell output for such position triggered sampling techniques can very often introduce errors as throughput times (i.e., package traveling speeds) vary. Such triggering devices very often are able to accomodate only certain ranges of package traveling speeds.
An object of this invention is to provide an automatic check weigher which samples weigh cell output independent of package position with regard to a specific, single point on a weighing platter.
A second object of this invention is to provide such an automatic check weigher which samples load cell output during impulse and oscillation periods as a package travels across the weighing platter.
A further object of this invention is to provide this check weigher with the capability of analyzing multiple samplings to determine three periods for each weighing operation: an initial impulse package load period, a package-on-the-weighing platter period, and a package discharging and weight display period.
An even further object of this invention is to provide this check weigher with the ability to calculate an average weight from multiple samples taken during the package-on-the-weighing platter period.
Additionally, a further object of this invention is to provide this check weigher with an automatic calibrating capability including dynamic sensing as well as to provide it optional multiple calibration run capability and keyboard data entry capability.